1. Field of the Invention
In general, the present invention relates to the production of zeolites.
2. Description of the Prior Art
Certain naturally occurring hydrated metal aluminum silicates are called zeolites. The synthetic zeolites of the invention have compositions similar to some of the natural zeolites. The most common of these zeolites are sodium zeolites.
Zeolites consist basically of a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra. The tetrahedra are cross-linked by the sharing of oxygen atoms so that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to two or O/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, a sodium ion. This balance may be expressed by the formula, Al/Na=1. The spaces between the tetrahedra are occupied by water molecules prior to dehydration.
Zeolites may be activated by heating to effect the loss of the water of hydration. The dehydration results in crystals interlaced with channels of molecular dimensions that offer very high surface areas for the adsorption of foreign molecules. The interstitial channels of zeolite X are of a size such that heptacosafluorotributylamine and larger molecules will not enter into the channels. The interstitial channels of zeolite A will not accept molecules larger than 5.5 A.
Zeolites A and X may be distinguished from other zeolites and silicates on the basis of their x-ray powder diffraction patterns and certain physical characteristics. The x-ray patterns for several of these zeolites are described below. The composition and density are among the characteristics which have been found to be important in identifying these zeolites.
The basic formula for all crystalline sodium zeolites may be represented as follows:
Na.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2 :yH.sub.2 O PA1 1.0.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :1.85.+-.0.5SiO.sub.2 :yH.sub.2 O
In general, a particular crystalline zeolite will have values for x and y that fall in a definite range. The value x for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. Minor variations in the relative numbers of these atoms do not significantly alter the crystal structure of physical properties of the zeolite. For zeolite X, an average value for x is about 2.5 with the x value normally falling within the range 2.5.+-.0.5. For zeolite A, the x value normally falls within the range 1.85+0.5.
The value of y is not necessarily an invariant for all samples of zeolites. This is true because various exchangeable ions are of different size, and, since there is no major change in the crystal lattice dimensions upon ion exchange, the space available in the pores of the zeolite to accommodate water molecules varies.
The average value for y determined for zeolite X is 6.2. For zeolite A, it is 5.1.
In zeolites synthesized according to the preferred procedure, the molar ratio Na.sub.2 O/Al.sub.2 O.sub.3 should equal one. But if all the excess sodium present in the mother liquor is not washed out of the precipitated product, analysis may show a ratio greater than one, and if the washing is carried too far, some sodium may be ion exchanged by hydrogen, and the ratio will drop below one. It has been found that due to the ease with which hydrogen exchange takes place, the ratio for zeolite X lies in the range of EQU Na.sub.2 O/Al.sub.2 O.sub.3 =0.9.+-.0.2.
The ratio for zeolite A lies in the range of EQU Na.sub.2 O/Al.sub.2 O.sub.3 =1.0.+-.0.2.
Thus, the formula for zeolite A may be written as follows:
The formula for zeolite X may be written as follows:
0.9.+-.0.2Na.sub.2 O:Al.sub.2 O.sub.3 :2.5.+-.0.5SiO.sub.2 :yH.sub.2 O
"y" May be any value up to 6 for zeolite A; any value up to 8 for zeolite X.
The pores of zeolites normally contain water.
The above formulas represent the chemical analysis of zeolites A and X. When other materials as well as water are in the pores, chemical analysis will show a lower value of y and the presence of other adsorbates. The presence in the crystal lattice of materials volatile at temperatures below about 600.degree. C. does not significantly alter the usefulness of the zeolites as an adsorbent since the pores are usually freed of such volatile materials during activation.
Among the ways of identifying zeolites and distinguishing them from other zeolites and other crystalline substances, the X-ray powder diffraction pattern has been found to be a useful tool. In obtaining the X-ray powder diffraction patterns, standard techniques are employed. The radiation is the K.alpha. doublet of copper, and a Geiger counter spectrometer with a strip chart pen recorder is used. The peak heights, I, and the positions as a function of 2.theta. where .theta. is the Bragg angle, were read from a spectrometer chart. From these, the relative intensities, 100 I/I.sub.o, where I.sub.o is the intensity of the strongest line or peak, and d the interplanar spacing in A corresponding to the recorded lines were calculated.
X-ray powder diffraction data for sodium zeolite X are given in Table A. 100 I/I.sub.o and the d values in angstroms (A) for the observed lines for zeolite X are also given. The X-ray patterns indicate a cubic unit cell of dimensions between 24.5 A and 25.5 A. In a separate column are listed the sum of the squares of the Miller indices (h.sup.2 +k.sup.2 +l.sup.2) for a cubic unit cell that corresponds to the observed lines in the x-ray diffraction patterns. The a.sub.o value for zeolite X is 24.99 A where a.sub.o is the unit cell edge.
Zeolite X is a name given to a synthetic zeolite having the crystal structure of the naturally occurring mineral, faujasite. Zeolite X is the name for those compounds having an SiO.sub.2 /Al.sub.2 O.sub.3 ratio of less than 3.
TABLE A ______________________________________ X-RAY DIFFRACTION PATTERN FOR ZEOLITE X h.sup.2 + k.sup.2 + l.sup.2 ##STR1## d (A) ______________________________________ 3 100 14.47 8 18 8.85 11 12 7.54 19 18 5.73 27 5 4.81 32 9 4.42 35 1 4.23 40 4 3.946 43 21 3.808 44 3 3.765 48 1 3.609 51 1 3.500 56 18 3.338 59 1 3.253 67 4 3.051 72 9 2.944 75 19 2.885 80 8 2.794 83 2 2.743 88 8 2.663 91 3 2.620 96 1 2.550 108 5 2.404 123 1 2.254 128 3 2.209 131 3 2.182 136 2 2.141 139 2 2.120 144 1 2.083 164 1 1.952 168 1 1.928 184 1 1.842 195 1 1.789 200 2 1.767 211 3 1.721 243 3 1.603 ______________________________________
The more significant d values for zeolite X are given in Table B.
TABLE B ______________________________________ MOST SIGNIFICANT d VALUES FOR ZEOLITE X d Value of Reflection in A ______________________________________ 14.42 .+-. 0.2 8.82 .+-. 0.1 4.41 .+-. 0.05 3.80 .+-. 0.05 3.33 .+-. 0.05 2.88 .+-. 0.05 2.79 .+-. 0.05 2.66 .+-. 0.05 ______________________________________
X-ray powder diffraction data for sodium zeolite A are given in Table C.
TABLE C ______________________________________ X-RAY DIFFRACTION PATTERN FOR ZEOLITE A h.sup.2 + k.sup.2 + l.sup.2 d (A) ##STR2## ______________________________________ 1 12.29 100 2 8.71 70 3 7.11 35 4 6.15 2 5 5.51 25 6 5.03 2 8 4.36 6 9 4.107 35 10 3.895 2 11 3.714 50 13 3.417 16 14 3.293 45 16 3.078 2 17 2.987 55 18 2.904 10 20 2.754 12 21 2.688 4 22 2.626 20 24 2.515 6 25 2.464 4 26 2.414 &gt;1 27 2.371 3 29 2.289 1 30 2.249 3 32 2.177 7 33 2.144 10 34 2.113 3 35 2.083 4 36 2.053 9 41 1.924 7 42 1.901 4 44 1.858 2 45 1.837 3 49 1.759 2 50 1.743 13 53 1.692 6 54 1.676 2 55 1.661 2 57 1.632 4 59 1.604 6 ______________________________________
The more significant d values for zeolite A are given in Table D.
TABLE D ______________________________________ MOST SIGNIFICANT d VALUES FOR ZEOLITE A d Value of Reflection in A ______________________________________ 12.2 .+-. 0.2 8.7 .+-. 0.2 7.10 .+-. 0.15 5.50 .+-. 0.10 4.10 .+-. 0.10 3.70 .+-. 0.07 3.40 .+-. 0.06 3.29 .+-. 0.05 2.98 .+-. 0.05 2.62 .+-. 0.05 ______________________________________
Occasionally, additional lines not belonging to the pattern for the zeolite appear in a pattern along with the X-ray lines characteristic of that zeolite. This is an indication that one or more additional crystalline materials are mixed with the zeolite in the sample being tested. Frequently these additional materials can be identified as initial reactants in the synthesis of the zeolite, or as other crystalline substances. When the zeolite is heat treated at temperatures of between 100.degree. C. and 600.degree. C. in the presence of water vapor or other gases or vapors, the relative intensities of the lines in the X-ray pattern may be appreciably changed from those existing in the unactivated zeolite patterns. Small changes in line positions may also occur under these conditions. These changes in no way hinder the identification of these X-ray patterns as belonging to the zeolite.
The particular X-ray technique and/or apparatus employed, the humidity, the temperature, the orientation of the powder crystals and other variables, all of which are well known and understood to those skilled in the art of X-ray crystallography or diffraction can cause some variations in the intensities and positions of the lines. These changes, even in those few instances where they become large, pose no problem to the skilled X-ray crystallographer in establishing identities. Thus, the X-ray data given herein to identify the lattice for a zeolite are not to exclude those materials which, due to some variable mentioned or otherwise known to those skilled in the art, fail to show all of the lines, or show a few extra ones that are permissible in the cubic system of that zeolite, or show a slight shift in position of the lines, so as to give a slightly larger or smaller lattice parameter.
A simple test described in "American Mineralogist", Vol. 28, page 545, 1943, permits a quick check of the silicon to aluminum ratio of the zeolite. According to the description of the test, zeolite minerals with a three-dimensional network that contains aluminum and silicon atoms in an atomic ratio of Al/Si=2/3=0.67, or greater, produce a gel when treated with hydrochloric acid. Zeolites having smaller aluminum to silicon ratios disintegrate in the presence of hydrochloric acid and precipitate silica.
U.S. Pat. No. 2,882,243 describes a process for making zeolite A comprising preparing a sodium-aluminum-silicate water mixture having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of from 0.5:1 to 2.5:1, an Na.sub.2 O/SiO.sub.2 mole ratio of from 0.8:1 to 3:1, and an H.sub.2 O/Na.sub.2 O mole ratio of from 35:1 to 200:1, maintaining the mixture at a temperature of from 20.degree. C. to 175.degree. C. until zeolite A is formed, and separating the zeolite A from the mother liquor.
U.S. Pat. No. 2,882,244 describes a process for making zeolite X comprising preparing a sodium-aluminum-silicate water mixture having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of from 3:1 to 5:1, an Na.sub.2 O/SiO.sub.2 mole ratio of from 1.2:1 to 1.5:1, and an H.sub.2 O/Na.sub.2 O mole ratio of from 35:1 to 60:1, maintaining the mixture at a temperature of from 20.degree. C. to 120.degree. C. until zeolite X is formed and separating the zeolite X from the mother liquor.
The process described in U.S. Pat. No. 3,101,251 is similar to that described in U.S. Pat. Nos. 2,882,243 and 2,882,244, except that the reaction mixture contains an admixture of non-kaolinitic alumino-silicate mineral and sodium hydroxide that has been fused at a temperature of between 330.degree. C. and 370.degree. C.
In U.S. Pat. No. 3,119,659, a kaolin clay and sodium hydroxide are formed into a compact body, dried, reacted in an aqueous mixture at a temperature of from 20.degree. C. to 175.degree. C. until a zeolite is formed. Zeolite A is formed in a reaction mixture having an Na.sub.2 O/SiO.sub.2 molar ratio of 0.5:1 to 1.5:1, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 1.6:1 to 2.4:1 and an H.sub.2 O/Na.sub.2 O molar ratio of 20:1 to 100:1. Zeolite X is formed in a reaction mixture having an Na.sub.2 O/SiO.sub.2 molar ratio of 1.5:1, an SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of 5:1, and an H.sub.2 O/Na.sub.2 O molar ratio of 30:1 to 60:1.
U.S. Pat. No. 4,235,856 discloses a process for making a combination of zeolite X and zeolite A wherein a sodium aluminate solution is added to a sodium silicate solution to form a mixture, then heating and reacting the mixture to 80.degree.-120.degree. C. until the combination zeolite is formed.
Another patent which discloses a process for making synthetic zeolite particles having two different sized effective pore entrance diameters within a single particle, namely a particle containing both a Type A zeolite structure and a Type X zeolite structure is U.S. Pat. No. 3,366,578.
U.S. Pat. No. 4,094,778 discloses a process for sequestering calcium and magnesium cations using mixtures of zeolite A and zeolite X.
British Pat. No. 1,533,496 sets forth a process for preparing low silica faujasite-type zeolites by adding potassium hydroxide or a potassium salt to the alumina trihydrate in addition to the sodium hydroxide prior to the addition of the sodium silicate.
U.S. Pat. No. 4,166,099 discloses a method for preparing crystalline aluminosilicates, such as a Type X synthetic faujasite by seeding an alkaline precursor mixture of alumina and silica with small size zeolite seeds having an average particle size below about 0.1 micron.
Zeolites are useful as molecular sieves and as sequestering agents for calcium and magnesium cations. They are particularly useful in detergent or washing compositions.
It is a primary object of the present invention to provide a faster and more economical process for making combination zeolite X and zeolite A particles.